The Files app cannot access images in the Photos app or music in the Music app. The only way to add music to the Music app is to copy the files onto the iPhone from a computer. You can however install VLC player and copy the files into the VLC folder. I guess VLC player is more trustworthy than Apple Music considering it's less isolated. Or Apple really wants you to pay the Music subscription, who knows. Want to give another app access to these files? You'll have to duplicate them, using up more storage space.

I get that it's supposedly about security, but this is not the only secure way. It is however the most convenient secure way for Apple, as now the only simple method of backing up and syncing files through all those isolated containers is iCloud.